1. Field of the Invention
The present invention relates to a light detecting apparatus and an optical pickup apparatus.
2. Description of the Related Art
Currently, CD (Compact Disc), DVD (Digital Versatile Disc), etc., are widely used as an optical disc medium for recording/reproducing information. To record/reproduce information on/from the optical disc medium, a focal point of a laser beam condensed by an object lens must be accurately focused on an information surface of the optical disc medium. Therefore, the focusing control is performed for correcting an error when the focal point of the laser light does not focused on the information surface of the optical disc medium, and the tracking control is performed for correcting an error when a spot condensed on the information surface is misaligned from the center of the predetermined track.
For example, the focusing control using the differential astigmatic method is known. In the focusing control using the differential astigmatic method, first, for example, 0th order light and ±1st order light are generated by diffracting the laser beam with a diffraction grating, etc. The 0th order light and the ±1st order light are applied to the information surface of the optical disc medium. To the reflected light of the 0th order light and ±1st order light reflected by the information surface, astigmatism is added by a cylindrical lens, etc. The reflected light of the 0th order light and ±1st order light with the astigmatism added is received by three four-split photodetectors disposed on a light detecting apparatus. FIG. 7 depicts a light-receiving pattern when the four-split photodetectors receive the reflected light of the 0th order light and ±1st order light while the focal point of the 0th order light is focused on the information surface of the optical disc medium. FIG. 8 depicts a light-receiving pattern when the four-split photodetectors receive the reflected light of the 0th order light and 1st order light, which has a maximum length defined as diagonal lines of light-receiving areas A′ to L′, while the focal point of the 0th order light is not focused on the information surface of the optical disc medium. Dash lines shown in FIGS. 7 and 8 show the light-receiving patterns of the reflected light of the 0th order light; dot-and-dash lines show the light-receiving patterns of the reflected light of the ±1st order patterns of the reflected light of the −1st order light. As shown in FIG. 7, when the focal point of the 0th order light is focused on the information surface of the optical disc medium, the light-receiving pattern of the reflected light of the 0th order light has a circular shape and is evenly received by the light-receiving areas A′ to D′. The light-receiving pattern of the reflected light of the −1st order light also has a circular shape and is evenly received by the light-receiving areas I′ to L′, and The light-receiving pattern of the reflected light of the −1st order light also has a circular shape and is evenly received by the light-receiving areas E′ to H′.
However, when the focal point of the 0th order light is not focused on the information surface of the optical disc medium, the light-receiving patterns of the reflected light of the 0th order light and ±1st order light have elliptical shapes, the diagonal lines of light-receiving areas A′ to D′, E′ to H′, and I′ to L′ corresponding to the respective centers of the elliptical shapes, and are not received evenly by the light-receiving areas A′ to L′. Based on the output from the light-receiving areas A′ to L′, a focus effort signal (hereinafter, FE signal) is generated by calculating {(output of light-receiving area A′+output of light-receiving area C′)−(output of light-receiving area B′+output of light-receiving area D′)}+k[{((output of light-receiving area I′+output of light-receiving area K′)−(output of light-receiving area J′+output of light-receiving area L′)}+{(output of light-receiving area E′+output of light-receiving area G′)−(output of light-receiving area F′+output of light-receiving area H′)}] (where k is the light intensity of the 0th order light/the light intensity of the ±1st order light). Based on the FE signal, the focusing control can be performed to focus the focal point of the 0th order light on the information surface of the optical disc medium.
Recently, a two-layer optical disc medium having two information surfaces is widely used as the optical disc medium. As shown in FIG. 9, the two-layer optical disc medium is configured by bonding a substrate with a first information surface L0 formed and a substrate with a second information surface L1 formed via an intermediate layer. The information surface L0 is configured by a translucent reflecting film, reflects partial amounts of the 0th order light and the ±1st order light, and transmits remaining amounts of the 0th order light and the ±1st order light. The information surface L1 is configured by a reflecting film, and reflects 0th order light and ±1st order light emitted from the information surface L0. The focusing control is also performed for the two-layer optical disc medium to focus the focal point of the 0th order light on the information surfaces L0 and L1 (Japanese Patent Application Laid-Open Publication No. 4-168631).
However, in the two-layer optical disc medium, for example, when the focal point of the 0th order light focused on the information surface L0 is focused on the information surface L1 (state of the 0th order light of FIG. 9), the reflected light of the 0th order light maybe applied to the light-receiving areas I′ and G′, which normally should receive the reflected light of the ±1st order light, as shown in FIG. 10. In such a case, the FE signal based on the output of the four-split photodetectors maybe affected. The impact on the FE signal will be described with reference to FIG. 11. A thin solid line of FIG. 11 shows a 0th order light FE signal [={(output of light-receiving area A′+output of light-receiving area C′)−(output of light-receiving area B′+output of light-receiving area D′)}] based on the output of the four-split photodetector receiving the reflected light of the 0th order light. A dash line shows ±1st order light FE signal [=k {(output of light-receiving area I′+output of light-receiving area K′)−(output of light-receiving area J′+output of light-receiving area L′) }] based on the output of the four-split photodetector receiving the reflected light of the +1st order light (or −1st order light). A heavy solid line shows the FE signal (=above equation) based on three four-split photodetectors. As shown in FIG. 11, an S-shape (with in a dot-and-dash line) not generated in the 0th order light FE signal is generated in the ±1st order light FE signal near the middle of the information surface L0 and the information surface L1. Since this S-shaped ±1st order light FE signal is generated, an S-shape is also generated in the FE signal. Especially, when the light intensity of the 0th order light is high as compared to the light intensity of the ±1st order light (i.e., the value of k is high), the outputs of the light-receiving areas I′ and G′ are amplified with a high gain and the S-shape is certainly generated. The generation of this S-shape may have an impact when the focal point of the 0th order light focused on the information surface L0 is focused on the information surface L1 (e.g., the 0th order light cannot accurately be focused on the information surface L1). When the focal point of the 0th order light focused on the information surface L1 is focused on the information surface L0, the reflected light of the 0th order light is applied to the light-receiving areas J′ and H′, which normally should receive the reflected light of the ±1st order light, and this may also have an impact when the focal point of the 0th order light focused on the information surface L1 is focused on the information surface L0.
In the two-layer optical disc medium, for example, when the focal point of the 0th order light is focused on the information surface L0, the reflected light of the 0th order light from the information surface L1 (hereinafter, stray light) may be applied to the light-receiving areas A′ and L′ as shown in FIG. 12. Therefore, this may have an impact on a signal process based on the reflected light of the 0th order light from the information surface L0 (e.g., deterioration of jitters) and an impact on the tracking control (e.g., offset of the signal based on the reflected light of the list order light in the case of using the differential push-pull method for the tracking control).